One of the most exciting aspects of Gege Akutami's Jujutsu Kaisen is the amount of intrigue in its world, story and characters. Many of its characters are shrouded in mystery, like Master Tengen, Yuki Tsukomo, and even the main antagonist, Ryomen Sukuna. The King of Curses was known as a frightening sorcerer during the "golden age of jujutsu." After becoming a Cursed Spirit, he continued to make a name for himself.
Akutami has kept much of Sukuna's past, abilities and motivations under wraps, only offering small nuggets of detail every so often. However, Sukuna's interest in Megumi Fushiguro is something fans have been curious about since its first mention. While the truth is yet to be revealed, let's look back at Sukuna and Fushiguro's interactions and explore what the King of Curses could be after.
There have been two significant interactions been Fushiguro and Sukuna, with the first instance occurring back in Jujutsu Kaisen Chapter #9 during the "Cursed Womb" arc. Holding Yuji captive by ripping out his heart, Sukuna then decides to fight Fushiguro, which quickly becomes a one-sided ordeal in the Curse's favor. However, it is here that Sukuna's interest in Fushiguro begins.
After a quick barrage of powerful attacks, Sukuna seemed content to observe Fushiguro, and more importantly, his Cursed Technique. Sukuna quickly deduced how Fushiguro's technique operated, understanding its potential. He then commented that Fushiguro was a "wasted treasure," having fled from the Special Grade cursed womb, seemingly admonishing him for running away. Yet, when Fushiguro prepares to use his final trump card, Sukuna's interest is genuinely piqued -- sensing something as the Jujutsu Sorcerer gathered his Cursed Energy.
This panel is referenced once again by Sukuna in Jujutsu Kaisen Chapter #118 during his fight with the Mahoraga -- the most powerful Shikigami of the Ten Shadows Technique. Fushiguro summoned Mahoraga as a last-ditch maneuver, and it would have cost him his life if Sukuna hadn't arrived to heal him. While Mahoraga's adaptive abilities gave Sukuna immense difficulties, he successfully defeated it to protect Fushiguro.
Though Sukuna clearly saw something in Fushiguro that Akutami is keeping in their back pocket, it likely ties to the young sorcerer's Zenin lineage or an element of his technique he's yet to have unlocked. This point was strengthened during Yuji's fight with Sukuna in his innate domain as the Curse mentioned that they were about to see something interesting, just as a panel of Fushiguro is shown.
Gege Akutami's lack of clarity on the subject has not stopped fans from speculating as many have gone to the internet, posting their theories regarding Fushiguro and Sukuna. One common theme surrounds the thought that Sukuna could be looking for a new body since he cannot control Yuji's fully. To do so, Sukuna may be looking to Fushiguro's technique as a way to create a new vessel. After all, he specifically mentions that he needs to keep Fushiguro alive as he needs him to do something for him.
During the "Shibuya Incident" arc, Sukuna temporarily gained control of Yuji's body after Jogo force-fed him ten fingers. He was given the opportunity to create a binding vow with Yuji to control the body entirely. However, he declined, stating that he had his own plans. This does seem to lend credibility to the idea that he is looking for a new host, as he told his servant, Uraume, that it would not be much longer until he was completely free.
While the exact nature of Sukuna's plan may be shrouded in mystery, it is pretty clear that Megumi Fushiguro will be a crucial figure. Though the initial fascination seemed born from dangerous curiosity surrounding Fushiguro's potential as a sorcerer, the most evident piece of information we know is that Sukuna needs him for an important task -- perhaps to create a new vessel or to summon something else entirely.